Fluorination pre-treatment of heat spreader attachment indium thermal interface material

ABSTRACT

The formation of electronic assemblies including a heat spreader coupled to a die through a thermal interface material formed from an indium preform, is described. One embodiment relates to a method including providing a preform comprising indium, the preform including an indium oxide layer thereon. The method also includes exposing the preform to fluorine so that part of the indium oxide layer is transformed into an indium oxy-fluoride. The method may also include, after the exposing the preform to fluorine so that part of the indium oxide layer is transformed into an indium oxy-fluoride, positioning the preform between a die and a heat sink, and applying pressure to and heating the preform positioned between the die and the heat sink so that reflow occurs and a bond is formed between the die and the heat sink.

RELATED ART

Integrated circuits may be formed on semiconductor wafers that areformed from materials such as silicon. The semiconductor wafers areprocessed to form various electronic devices thereon. The wafers arediced into semiconductor chips, which may then be attached to a packagesubstrate using a variety of known methods.

Operation of the integrated circuit generates heat in the device. As theinternal circuitry operates at increased clock frequencies and/or higherpower levels, the amount of heat generated may rise to levels that areunacceptable unless some of the heat can be removed from the device.Heat is conducted to a surface of the chip (also known as a die), andshould be conducted or convected away to maintain the temperature of theintegrated circuit below a predetermined level for purposes ofmaintaining functional integrity of the integrated circuit.

One way to conduct heat from a die is through the use of a heatspreader, which is a body thermally coupled to the die. The heatspreader may be positioned above the die and thermally coupled to thedie through a thermal interface material. Materials such as certainsolders may be used as a thermal interface material and to couple theheat spreader to the die. A flux is typically applied to at least one ofthe surfaces to be joined and the surfaces brought into contact. Theflux acts to remove the oxide on the solder surfaces to facilitatesolder wetting. The thermal interface material may be initially be asolid perform that is positioned between the heat spreader and die. Aheating operation at a temperature greater than the melting point of thethermal interface material is carried out, and a connection is madebetween the die and the heat spreader through the thermal interfacematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described by way of example, with reference to theaccompanying drawings, which are not drawn to scale, wherein:

FIG. 1 illustrates an indium preform having a native oxide thereon, inaccordance with certain embodiments;

FIG. 2 illustrates treating the indium preform of FIG. 1 with fluorineatoms, in accordance with certain embodiments;

FIG. 3 illustrates the formation of oxy-fluoride regions in the nativeoxide of the indium preform, in accordance with certain embodiments;

FIG. 4 illustrates a treated indium preform positioned between a die anda heat spreader, in accordance with certain embodiments;

FIG. 5 illustrates the formation of a joint with an indium thermalinterface material positioned between a die and a heat spreader, inaccordance with certain embodiments;

FIG. 6 is a flow chart of certain operations for treating a thermalinterface material perform and forming an assembly including a heatspreader bonded to at least one die through the thermal interfacedmaterial, in accordance with certain embodiments;

FIG. 7(A) illustrates an instrument for treating an indium preform, inaccordance with certain embodiments;

FIG. 7(B) illustrates a portion of an indium preform that may beprocessed in the instrument illustrated in FIG. 7(A), in accordance withcertain embodiments;

FIG. 8 illustrates an electronic system arrangement in which certainembodiments may find application.

DETAILED DESCRIPTION

Certain embodiments relate to the formation of electronic assemblies.Certain embodiments also relate to the pre-treatment of an indiumthermal interface material. Certain embodiments also relate to afluxless attach processes for forming connections between a die and aheat spreader.

FIG. 1 illustrates an indium body 10, also known as a preform, which maybe used as a thermal interface material in accordance with certainembodiments. The indium preform 10 may in certain embodiments includes acore region of indium (In) and a native oxide layer 14 on its surfaceformed from exposure of the indium to oxygen. The oxide layer 14protects the core region 12 from further oxidation. The oxide layer 14is strongly bound to the underlying core region 12. As a result, whilecarrying out heating of the indium preform 10 during a reflow operationfor attaching a heat spreader to a die, during the transition fromsolidus to liquidus of the indium preform 10, the native oxide layer 14maintains its solid state, creating a barrier between the liquid indiumand the surfaces it needs to bond to. To overcome this problem, avariety of chemical agents may be used as fluxes to remove the nativeoxide layer and promote bonding. The volatiles present in these fluxeshave been identified as a principal source of voids created during thereflow operation and thus the fluxes are responsible for an inefficienttransfer of thermal energy from the active areas of the die to thethermal heat spreader.

FIG. 2 illustrates an embodiment in which the indium body 10 is treatedprior to placing the indium body between a die and a heat spreader. Thetreatment includes exposure to highly reactive fluorine (F) species inatomic form 16, in a controlled vacuum atmosphere, such that thefluorine does not react with oxygen or hydrogen in an open atmosphere.Atoms of fluorine 16 are directed towards the indium body 10. A varietyof fluorine sources may be used. Due to the toxicity of fluorine, benignfluoride gases, for example, CF₄ and SF₆, may be used to initiate theprocess. These gases are used to generate the fluorine atoms in thecontrolled environment, the release being activated by, for example, amicrowave induced plasma.

Exposing the indium oxide to the fluorine results in the formation of anoxy-fluoride on the surface. In certain embodiments, the treatment iscontrolled so that at least part of the native oxide is transformed intoan oxy-fluoride and a portion of the native oxide is not transformed.FIG. 3 illustrates an embodiment including an indium preform 10including the formation of areas of oxy-fluoride 18 within the nativeoxide layer 14. As seen in FIG. 3, the areas of oxy-fluoride 18 may besurrounded by native oxide 14. The oxy-fluoride 18 has a relatively highmelting temperature but is brittle. As a result, when a suitable forceis applied, at a temperature lower that its melting point, theoxy-fluoride can be broken (due to its brittleness), and the indium core12 positioned under the oxy-fluoride 18 can then be exposed.

In certain embodiments, it is believed that a native oxide layer willreform on an oxy-fluoride region within about a week when stored in anair environment. When stored in an inert environment, it is believedthat it will take a longer time for a native oxide to form on theoxy-fluoride region. For example, in a nitrogen environment, it isbelieved that a native oxide layer will form on the oxy-fluoride regionwithin about two weeks. Thus, in such embodiments, the use of the indiumpreform as a thermal interface material, to couple a heat spreader to adie, should be carried out within these times.

As illustrated in FIG. 4, the indium preform 10 having the oxy-fluoride18 on the surface may be positioned between a die 20 and a heat spreader22. The heat spreader 22 may be formed from a variety of materials,including, but not limited to, copper (Cu), and in certain embodimentsmay include one or more metallization layers formed thereon, such asnickel, which may act as a wetting layer, and gold, which may act toprotect the nickel layer from oxidation.

The die 20 may in certain embodiments have a flip-chip configurationwith an active die surface facing a package substrate 24 and a back sidesurface facing the indium preform 10 (the thermal interface material).The back side surface of the die 20 may include a suitable back sidemetallization (BSM) that may provide oxidation protection and promotethe bonding of the die 20 to the indium thermal interface material. Incertain embodiments, the back side metallization includes one or moresuitable metal layers, for example, titanium (Ti), nickel (Ni) or nickelvanadium (NiV), and gold (Au).

The die 20 may be coupled to the package substrate 24 through, forexample, solder bumps 26, and a suitable die underfill material 31, forexample, a curable epoxy, may be present. A sealant material 28, whichmay in certain embodiments be formed from a polymer, may also be formedon the package substrate 24 surface. As illustrated in FIG. 4, the heatspreader 26 may include leg regions 29, 30 that will be positioned onthe sealant 32 to form a lid over the die 20 coupled to the substrate24.

A suitable clip mechanism 38 may be used to hold and apply a suitableforce F to the heat spreader 22, package substrate 24, and the indiumpreform 10 positioned therebetween during the heating operation. Incertain embodiments, the clamp or clip mechanism 38 is coupled to acarrier (not shown) which holds the substrate 24. The assembly is thenheated to a temperature sufficient to reflow the indium core 12. Incertain embodiments, the reflow operation can be conducted either in astandard air atmosphere or a nitrogen controlled atmosphere. The reflowof the indium preform 10 may in certain embodiments be carried out at atemperature that is lower than the melting point of the solder bumps 26.

It is believed that the combination of the heat and pressure breaks downthe brittle oxy-fluoride regions 18 and permits the indium core 12 ofthe indium preform 10 to flow and wet the surfaces of the die 20 andheat spreader 22 to form a strong bond therebetween. A flux need not beused, so void formation from flux residue is inhibited. FIG. 5illustrates an assembly after the heating operation including the heatspreader 22 coupled to the die 24 through the thermal interface material10′, which includes the reflowed indium preform 10. The joint betweenthe thermal interface material 10′ and the thermal heat spreader 22, andthe joint between the thermal interface material 10′ and the die 20, mayinclude material from the indium preform 10 and material from any of thevarious layer(s) on the heat spreader 22 and die 20, as describe above.Depending on the elements used in the various layers, the finished jointmay include a number of layers, including various combinations of theelements used. Some of the combinations may comprise alloys and some maycomprise intermetallic compounds. For example, where indium is used inthe thermal interface material, and one or more gold layers are used,the joint will in certain embodiments include one or more indium-goldalloys and one or more indium-gold intermetallic compounds. Assembliesincluding a substrate, die, thermal interface material and thermalspreader formed and joined together as described in embodiments abovemay find application in a variety of electronic components. Suchcomponents may include, but are not limited to, processors, controllers,chipsets, memory, and wireless devices.

FIG. 6 is a flow chart showing a number of operations in accordance withcertain embodiments. Box 150 is positioning in a vacuum chamber anindium preform having an indium core surrounded by a native oxide layer.Box 160 is treating the indium preform by exposing the indium preform tofluorine atoms, to transform at least a portion of the native oxidelayer to an oxy-fluoride layer. Box 170 is positioning the treatedindium preform between a heat spreader and one or more dies. The heatspreader is adapted to transmit heat away from the one or more dies,with the indium acting as a thermal interface material. Box 180 isapplying pressure and heat to the assembly including the indium preformpositioned between the heat spreader and die(s), so that theoxy-fluoride breaks down and indium in the core melts and wets the heatspreader and die to form a bond therebetween. A flux is not needed tomake the bond between the treated indium preform and the heat spreader.

FIG. 7(A) illustrates an example of a system 100 for performing afluorine treatment of one or more indium preforms 110, in accordancewith certain embodiments. The system 110 includes a vacuum chamber 112having an anode 114 and a cathode 116 positioned therein. In certainembodiments the anode may be formed from stainless steel and the cathodemay be formed from carbon. A power source such as an RF power source 118is connected to the anode 114 and cathode 116. A gas intake and flowregulator 120 may be positioned at a top portion of the system 110. Agas containing fluorine (for example, CF₄ and/or SF₆) is deliveredthrough the gas intake and flow regulator 120 and into the chamber 112.The gas may travel through apertures 126 extending through the anode 114and towards the middle of the chamber 112. A plasma may be generatedbetween the anode 114 and the cathode 116 in the plasma generationregion 122.

A plurality of indium preforms 110 are positioned on the cathode 116. Amasking cover 124 is positioned over the indium preforms 110. Themasking cover 124 may in certain embodiments be formed from stainlesssteel, and includes side supports 125 that are configured to permit gasto flow under the upper portion of the masking cover 124. The maskingcover 124 acts to inhibit the indium preforms 110 from direct contactwith the plasma. Reactive ions ejected from the plasma may reach thepreforms 110 from the sides, below the masking cover 124. Fluorine ionswill react with the indium oxide surface of the indium preforms 10 andform oxy-fluoride regions in the indium oxide surface. The system 110may in certain embodiments be configured so that the vacuum is generatedby pumping in a downward direction as indicated by the arrows extendingdownward through the bottom of the vacuum chamber 112 as illustrated inFIG. 7(A). Such a configuration enables gas atoms to make their way tothe bottom of the vacuum chamber 112 and be transmitted therefrom.

To permit both the top surface 119 and the bottom surface 121 of theindium preforms 110 to be reacted with the fluorine ions, in certainembodiments the indium preforms 110 have their corners bent asillustrated in FIG. 7(B), so that the four corners 111, 113, 115, 117act as legs and thus the bottom surface 19 is exposed. After thetreatment the corners may then be bent back to flatten the indiumpreform 110.

Assemblies including a substrate, die, thermal interface material andheat spreader formed and joined together as described in embodimentsabove may find application in a variety of electronic components. FIG. 8schematically illustrates one example of an electronic systemenvironment in which aspects of described embodiments may be embodied.Other embodiments need not include all of the features specified in FIG.8, and may include alternative features not specified in FIG. 8.

The system 201 of FIG. 8 may include at least one central processingunit (CPU) 203. The CPU 203, also referred to as a microprocessor, maybe a die which is attached to an integrated circuit package substrate205, which is then coupled to a printed circuit board 207, which in thisembodiment, may be a motherboard. The CPU 203 on the package substrate205 is an example of an electronic device assembly that may have astructure formed in accordance with embodiments such as described above.A variety of other system components, including, but not limited tomemory and other components discussed below, may also include assemblystructures formed in accordance with the embodiments described above.

The system 201 further may further include memory 209 and one or morecontrollers 211 a, 211 b . . . 211 n, which are also disposed on themotherboard 207. The motherboard 207 may be a single layer ormulti-layered board which has a plurality of conductive lines thatprovide communication between the circuits in the package 205 and othercomponents mounted to the board 207. Alternatively, one or more of theCPU 203, memory 209 and controllers 211 a, 211 b . . . 211 n may bedisposed on other cards such as daughter cards or expansion cards. TheCPU 203, memory 209 and controllers 211 a, 211 b . . . 211 n may each beseated in individual sockets or may be connected directly to a printedcircuit board. A display 215 may also be included.

Any suitable operating system and various applications execute on theCPU 203 and reside in the memory 209. The content residing in memory 209may be cached in accordance with known caching techniques. Programs anddata in memory 209 may be swapped into storage 213 as part of memorymanagement operations. The system 201 may comprise any suitablecomputing device, including, but not limited to, a mainframe, server,personal computer, workstation, laptop, handheld computer, handheldgaming device, handheld entertainment device (for example, MP3 (movingpicture experts group layer-3 audio) player), PDA (personal digitalassistant) telephony device (wireless or wired), network appliance,virtualization device, storage controller, network controller, router,etc.

The controllers 211 a, 211 b . . . 211 n may include one or more of asystem controller, peripheral controller, memory controller, hubcontroller, I/O (input/output) bus controller, video controller, networkcontroller, storage controller, communications controller, etc. Forexample, a storage controller can control the reading of data from andthe writing of data to the storage 213 in accordance with a storageprotocol layer. The storage protocol of the layer may be any of a numberof known storage protocols. Data being written to or read from thestorage 213 may be cached in accordance with known caching techniques. Anetwork controller can include one or more protocol layers to send andreceive network packets to and from remote devices over a network 217.The network 217 may comprise a Local Area Network (LAN), the Internet, aWide Area Network (WAN), Storage Area Network (SAN), etc. Embodimentsmay be configured to transmit and receive data over a wireless networkor connection. In certain embodiments, the network controller andvarious protocol layers may employ the Ethernet protocol over unshieldedtwisted pair cable, token ring protocol, Fibre Channel protocol, etc.,or any other suitable network communication protocol.

While certain exemplary embodiments have been described above and shownin the accompanying drawings, it is to be understood that suchembodiments are merely illustrative and not restrictive, and thatembodiments are not restricted to the specific constructions andarrangements shown and described since modifications may occur to thosehaving ordinary skill in the art.

1. A method for processing a body comprising indium, comprising:providing an body comprising indium, the body including an indium oxidelayer on a surface thereof; and exposing the body to fluorine andforming regions of indium oxy-fluoride on the surface.
 2. The method ofclaim 1, wherein the exposing the body to fluorine includes introducinga gas comprising fluorine into a plasma chamber, forming a plasma of thefluorine gas to generate fluorine ions, and directing a plurality of thefluorine ions towards the body.
 3. The method of claim 2, wherein thegas comprising fluorine is selected from the group consisting of CF₄ andSF₆.
 4. The method of claim 1, further comprising, after the exposingthe body to fluorine and forming regions of indium oxy-fluoride on thesurface; positioning the body between a chip and a heat sink; andheating the body, in the absence of a flux, and forming a bond betweenthe die and the heat sink.
 5. The method of claim 4, wherein the heatingthe body in carried out in air.
 6. The method of claim 4, wherein theheating the body in carried out in nitrogen.
 7. A method for forming anelectronic device, comprising: providing a preform comprising indium,the preform including an indium oxide layer thereon; exposing thepreform to fluorine so that part of the indium oxide layer istransformed into indium oxy-fluoride; after the exposing the preform tofluorine, positioning the preform between a die and a heat sink; andafter the positioning the preform between the die and the heat sink,applying pressure to and heating the preform so that reflow occurs and abond is formed between the die and the heat sink.
 8. The method of claim7, wherein prior to the positioning the preform between the chip and theheat sink, the chip is coupled to a substrate using a plurality ofsolder bumps; and wherein the heating the preform so that reflow occursis carried out at a temperature that is lower than the melting point ofthe solder bumps.
 9. The method of claim 8, wherein prior to thepositioning the preform between the chip and the heat sink, an epoxymaterial is positioned between the die and the substrate and cured. 10.The method of claim 8, wherein the heat sink is configured to be a lidthat covers the die.
 11. The method of claim 8, wherein the heat sink isincludes end regions, and method further comprises positioning a sealantmaterial between end regions of the heat sink and the substrate.
 12. Themethod of claim 7, wherein the heating the preform is carried out in theabsence of a flux.
 13. The method of claim 10, wherein the heating thepreform is carried out in at atmosphere selected from the groupconsisting of air and nitrogen.
 14. The method of claim 7, wherein theexposing the preform to fluorine includes introducing a gas comprisingfluorine into a plasma chamber, forming a plasma of the fluorine gas togenerate fluorine ions, and directing a plurality of the fluorine ionstowards the preform.
 15. The method of claim 12, wherein the gascomprising fluorine is selected from the group consisting of CF₄ andSF₆.
 16. A preform body comprising: a core comprising indium; and asurface comprising indium oxide and indium oxy-fluoride.
 17. The preformof claim 16, wherein the core is pure indium.
 18. The preform of claim16, wherein the core comprises an indium alloy.
 19. The preform of claim16, wherein the core comprises indium and tin.
 20. The preform of claim16, wherein the surface comprises regions of indium oxy-fluoridesurrounded by indium oxide.